Membranes for use within a lithographic apparatus and a lithographic apparatus comprising such a membrane

ABSTRACT

A membrane transmissive to EUV radiation, which may be used as a pellicle or spectral filter in a lithographic apparatus. The membrane has one or more high doped regions wherein the membrane is doped with a dopant concentration greater than 1017 cm−3, and one or more regions with low (or no) doping. The membrane may have a main substrate having low doping and one or more additional layers, wherein the high doped regions are within some or all of the additional layers.

This application is a continuation of U.S. patent application Ser. No.15/320,749, filed Dec. 20, 2016, which is the U.S. national phase entryof PCT patent application no. PCT/EP2015/065080, which was filed on Jul.2, 2015, which claims the benefit of priority of European patentapplication no. 14175835.9, which was filed on Jul. 4, 2014, and ofEuropean patent application no. 15169657.2, which was filed on May 28,2015, each of the foregoing applications is incorporated herein in itsentirety by reference.

FIELD

The present invention relates to membranes for use within a lithographicapparatus, and more specifically to EUV transmissive membranes which canform part of pellicle or optical filter components within the apparatus,and a lithographic apparatus comprising such a membrane.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

Lithography is widely recognized as one of the key steps in themanufacture of ICs and other devices and/or structures. However, as thedimensions of features made using lithography become smaller,lithography is becoming a more critical factor for enabling miniature ICor other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given bythe Rayleigh criterion for resolution as shown in equation (1):

$\begin{matrix}{{CD} = {k_{1}*\frac{\lambda}{NA}}} & (1)\end{matrix}$

where λ is the wavelength of the radiation used, NA is the numericalaperture of the projection system used to print the pattern, k1 is aprocess dependent adjustment factor, also called the Rayleigh constant,and CD is the feature size (or critical dimension) of the printedfeature. It follows from equation (1) that reduction of the minimumprintable size of features can be obtained in three ways: by shorteningthe exposure wavelength λ, by increasing the numerical aperture NA or bydecreasing the value of k1.

In order to shorten the exposure wavelength and, thus, reduce theminimum printable size, it has been proposed to use an extremeultraviolet (EUV) radiation source. EUV radiation is electromagneticradiation having a wavelength within the range of 5-20 nm, for examplewithin the range of 13-14 nm. It has further been proposed that EUVradiation with a wavelength of less than 10 nm could be used, forexample within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Suchradiation is termed extreme ultraviolet radiation or soft X-rayradiation. Possible sources include, for example, laser-produced plasmasources, discharge plasma sources, or sources based on synchrotronradiation provided by an electron storage ring or based on free electronlaser.

Thin transmissive EUV membranes are often required in EUV lithographicapparatus for a number of reasons. One such reason may be to protect,for example, reticles and/or lithographic components from contaminationby particles (with a grain size ranging from nm to μm). Another reasonmay be to spectrally filter out unwanted radiation wavelengths from thegenerated EUV radiation.

The transmissive EUV membranes (or shortly EUV membranes) are requiredto be highly transparent to EUV radiation, and therefore need to beextremely thin. Typical EUV membranes have a thickness of 10 to 100 nm,to minimize absorption of EUV radiation.

EUV membranes may comprise a free-suspended (i.e. self-standing)membrane (a film) comprising a material such as polysilicon (poly-Si),produced by etching of a silicon wafer. EUV membranes may also compriseone or more layers of protective coatings (e.g. protective cap layers)on one or both surfaces to prevent EUV-induced plasma etching (forexample induced by hydrogen (H, H⁺, H₂ ⁺and/or H₃ ⁺)).

Although absorption of EUV radiation by EUV membranes may be low, it isin practice still not zero and absorption of residual EUV radiationresults in an increase in temperature of the EUV membrane. Becausepellicles are in vacuum, the main process for pellicle cooling isradiative heat transfer. Should the temperature of an EUV membraneexceed a damage threshold (for example, about 500 to 700° C.), damage tothe EUV membrane may occur. Damage can also occur, or be amplified, whenthere are large temperature gradients within the EUV membrane. Wheresuch damage is severe, the EUV membrane may break, leading todamage/contamination of an unprotected reticle or other elements of thelithographic apparatus such as mirrors, or photoresist exposure toundesired non-EUV wavelength radiation, leading to a significantmanufacturing process downtime.

It is apparent that maintaining the temperature of the EUV membranebelow the damage threshold, as well as minimizing temperature gradients,can increase the EUV membrane lifetime.

The reason that pellicles may fail due to heat load is that they do notabsorb/emit IR radiation very well, especially for high power EUVradiation sources such as 125 Watt sources and beyond. Since thermalradiation is emitted in the IR wavelength region, a high spectral (IR)hemispherical emissivity enables a substantial heat loss for EUVmembranes. It is therefore desirable to manufacture EUV pellicles whichhave a high spectral emissivity. Also, EUV pellicles need to be verythin if a rage amount of EUV radiation such as 90% or more is to betransmitted through an EUV membrane.

SUMMARY

It is desirable to improve the thermal characteristics of EUV membranes,such as improved cooling and/or minimization of temperature gradientswithin the EUV membranes. Herein an EUV membrane means a membranesubstantially transmissive to EUV radiation and is also referred to asEUV pellicle. By substantially transmissive (or simply transmissive) toEUV radiation herein is meant to be transmissive for at least 65% EUVradiation, preferable for at least 75% EUV radiation, more preferably atleast 85% EUV radiation and most preferable at least 90% EUV radiationin order to provide sufficient EUV dose during exposure.

In order to increase an EUV pellicle emissivity to IR radiation whileEUV transmission is still substantial, it is herein proposed to:

a) dope the EUV pellicle with impurities; and/or

b) coat the EUV pellicle with a cap layer for improved IR emissivitycomprising a material which is good absorber for IR radiation buttransparent in the EUV radiation regime, for example with a metal caplayer. Such a cap layer preferably also will protect the pellicle fromoxidation or other environmental hazards. The EUV pellicle may be chosento be transmissive for 90% or more of a given EUV radiation wavelength,such as 13.5 nm or 6.8 nm (or any other EUV radiation wavelength).

Herein by improved (increased, enhanced, optimal) IR emissivity of a EUVmembrane according to the invention or EUV membrane assembly is meantthat the IR emissivity is more than 0.1, such as more than 0.15 andpreferably more than 0.2. Preferably the IR emissivity of the EUVmembrane is increased by at least a factor 2 for a given temperature.

If the EUV membrane (e.g., EUV pellicle) is formed by a core layer (alsoreferred to as a main substrate layer) and one or more cap layers (alsoreferred herein as cover layers, being generally layers with a specificfunctionality such as a protective cap layer) from which at least onecap layer having the function to improved IR emissivity, then by caplayer for improved IR emissivity is meant a cap layer with an IRemissivity selected such that the IR emissivity of the EUV membrane islarger than the IR emissivity of the core layer. For example, if the EUVemissivity of the core layer is about 0.1, then the material andthickness of the cap layer for improved IR emissivity is selected suchthat the total IR emissivity of the EUV membrane determined in sameconditions is more than 0.15. Although cap layer is mainly referredherein as a coating which may be provided on the top of a core layer, itis herein understood that a cap layer may also be a layer in between twocore layers, or in between core layer and another (second) cap layer, orbetween two cap layers of same or different functionality (e.g.protective such as anti-oxidant layer, anti-diffusive, or for improvedIR emissivity).

By core layer or main substrate layer herein is generally understood athicker layer, a multilayer stack or a layer of high yield strengthmaterial which also provides most of the mechanical strength for the EUVmembrane. For example, in order to withstand large stresses that mayoccur during exposure due to the high thermal load the core layer mayneed to have a yield strength of at least 50 MPa, preferably at least100 MPa, even more preferably at least 150 MPa. Generally, a yieldstrength in the range of 50 to 1000 MPa may provide sufficientmechanical strength to the EUV membrane depending on the material (forexample p-Si has about 180 MPa and SiNx has about 500 MPa yieldstrength) Generally, the thickness of the core layer may be larger thanthe thickness of the cap layer for improved emissivity. When the corelayer is formed by a multilayer stack, the total thickness of the stackmay be larger than the thickness of the cap layer for improvedemissivity, even though the thickness of individual layers in themultilayer stack may be comparable with the thickness of the cap layerfor improved emissivity. However, depending on the materials of the coreand cap layers, the EUV membrane can also be designed to have comparablethicknesses or even the cap layer for improved emissivity to be somewhatthicker than the core layer, as long as desired requirements for EUVtransmission, DUV suppression and/or IR emissivity are met.

If the EUV membrane (EUV pellicle) is doped to increase its emissivity,by improved IR emissivity of the EUV membrane is meant as the IRemissivity of the doped EUV membrane is larger than the IR emissivity ofthe non-doped EUV membrane of same material and thickness, at sameconditions.

In an alternative definition of the improved emissivity also thetemperature may be taken as defining parameter. For example, improvingthe IR emissivity of an EUV membrane can also be defined as increasingthe thermal emissivity of the EUV membrane for the wavelengths (forexample 1 to 10 μm) such that more than 65% (preferably more than 85%)of the energy absorbed by the EUV membrane is radiated away when thetemperature of the EUV membrane ranges from 100 to about 1000° C., andmore specifically at moderate temperatures (less than 500° C.).

By emissivity herein is generally meant the hemispherical emissivity(based on hemispherical IR radiation absorption), unless otherwisestated.

In an aspect of the invention there is provided a membrane transmissiveto EUV radiation which is doped with donor and/or acceptor impurities asto increase the IR emissivity of the EUV pellicle. It has been found thedoping range needed in order to match the plasmon resonance to the peakin the Planck spectrum as to maximize IR emissivity (i.e. The Planckemissivity peak corresponds to the plasmon resonance). For example, bydoping (poly-)silicon a plasmon in the IR is created at around 1-10microns, which coincides with the peak Planck black body radiation. Theplasmon frequency is easily determined from the number of conductors.The volume density of atoms is roughly 10²² n/cm³ for solids. In case ofa metal each atom contributes with an electron in the conduction band,resulting in about 10²² carriers/cm³. A metal has a typical plasmawavelength of the order of 150 nm.

The plasma frequency ω_(p) is proportional to ω_(p)=√n_(e) with n_(e)the free charge carrier. If a 10× larger wavelength is desired (i.e. thefrequency is 10× lower), a 100× lower free charge carrier density isneeded which corresponds to 10²⁰ carriers. It follows that if(poly-)silicon is doped with 0.1-10% atom concentration of a dopant,then a plasmon resonance may be created in IR radiation spectrum. Thisplasmon couples to the Planck black body spectrum and creates additionalIR absorption.

If the plasmon resonance frequency is much higher than the Planckfrequency (10 micron at 300 K), then the EUV pellicle may become morereflective. (i.e. metallic-like). If the plasmon resonance frequency ismuch lower than the Planck frequency then the EUV pellicle becomes moretransmissive (i.e. dielectric-like). The desired behavior for the EUVpellicle is a semi-metallic behavior where the plasma wavelength isbetween 1 and 10 microns.

From theoretical calculations it was found that an optimal IR emissivityof a 60 nm thick polysilicon pellicle is obtained with having N-typedoping of an EUV pellicle material with at least about (2 to3)×10²⁰n/cm³ donor atoms. The higher the pellicle temperature, thehigher the doping concentration should be due to the shift of Planckspectrum at higher temperatures. The optimal doping in case of P-typedoping of an EUV pellicle material was found to be at least 4×10²⁰n/cm³acceptor atoms. P-type doping results in slightly (about 10%) higher IRemissivity than N-type doping. Compared with a 60 nm thick polysiliconpellicle, a thinner pellicle would have a higher optimal dopingconcentration (e.g. 20 nm thick pellicle has optimal doping around 1e²¹)and a thicker pellicle would have a lower optimal doping concentration(200 nm thick Si pellicle has optimal doping around 1e²⁰ ). Generally,for an EUV pellicle with a thickness between 10 and 250 nm the optimaldopant concentration ranges from 5×10¹⁹ to 1×10²¹ n/cm³ atoms.

In an aspect of the invention there is provided a membrane transmissiveto EUV radiation comprising: one or more high doped regions where themembrane is doped with a high dopant concentration, and one or more lowdoped regions where the membrane has no doping or a low dopantconcentration; wherein a high dopant concentration is defined as dopantconcentration greater than 10¹⁷ cm⁻³, preferably greater than 10²⁰ cm⁻³; and a low dopant concentration is defined as a dopant concentrationless than 10¹⁷ cm⁻³, preferably less than 10²⁰ cm⁻³.

In another aspect of the invention there is provided a membranetransmissive to EUV radiation (EUV pellicle) having a (core) materialselected from (poly-)Si, Si₃N₄, SiC, ZrN, ZrB₂, ZrC, MoB₂, MoC, RuB₂,LaB₂, LaC, TiB₂, TiC, (poly-)crystalline Yttrium, (poly-) crystallineZr, Be, C, B and B₄C and composites or combinations of multilayerstherefrom. Semi-metals such as ZrB₂ or ZrC may reduce the electrostaticcharging of the EUV pellicle. The EUV pellicle has preferably athickness of 60 nm or less to allow sufficient EUV transmission.

In another aspect of the invention there is provided a membrane for alithographic apparatus having IR radiation emissivity of at least 0.1and being substantially transmissive to EUV radiation of 6.7 nmwavelength, the membrane comprising a core layer from a materialcomprising boron, wherein the core layer has a thickness from 20 to 150nm.

In another aspect of the invention there is provided a membrane for alithographic apparatus having IR radiation emissivity of at least 0.1and being substantially transmissive to EUV radiation, the membranecomprising a core layer from a material comprising Ru, wherein the corelayer has a thickness from 20 to 30 nm.

In another aspect of the invention there is provided a membrane assemblyfor a lithographic apparatus having IR radiation emissivity of at least0.1 and being substantially transmissive to EUV radiation, the membraneassembly comprising at least two independent metal layers for improvedIR emissivity, the metal layers comprising a metal which absorbs IRradiation and have a layer thickness of 20 nm or less such that they aresubstantially transparent for EUV, wherein the metal layers for improvedIR emissivity are separated by a gap with thickness D of 10 microns orless. The metal layers may be supported with a support layer whichprovides mechanical strength.

In another aspect of the invention there is provided a lithographicapparatus comprising one or more EUV membranes according to the aboveembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention. Embodiments of the invention are described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 depicts schematically a lithographic apparatus having reflectiveprojection optics;

FIG. 2 is a more detailed view of the apparatus of FIG. 1;

FIG. 3 illustrates an EUV membrane according to a first embodiment ofthe invention being used as a pellicle for a reticle;

FIG. 4 illustrates an EUV membrane according to a second embodiment ofthe invention;

FIG. 5 illustrates an EUV membrane according to a third embodiment ofthe invention;

FIG. 6 illustrates an EUV membrane according to a fourth embodiment ofthe invention;

FIG. 7 is a graph of expected temperature distribution against distanceL across an EUV membrane, for a flat EUV membrane and for the EUVmembrane depicted in FIG. 6;

FIG. 8 illustrates an EUV membrane according to a fifth embodiment ofthe invention;

FIG. 9 illustrates the emissivity of a poly-Si EUV membrane as functionof temperature for different doping concentrations;

FIG. 10 illustrates a comparison of EUV membrane power absorption andmaximum temperature vs EUV source power;

FIG. 11 shows the effect of IR emissivity on EUV membrane temperature;

FIG. 12 shows the effect of Ru cap layer for improved IR emissivitycompared with a (poly-)Si EUV membrane; and

FIG. 13 illustrates a dual EUV pellicle (i.e. a membrane assembly) whichenhances IR emissivity due to resonant absorption.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 schematically depicts a lithographic apparatus 100 including asource module SO according to one embodiment of the invention. Theapparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. EUV radiation).

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask or a reticle) MA and connected to a firstpositioner PM configured to accurately position the patterning device;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate; and

a projection system (e.g. a reflective projection system) PS configuredto project a pattern imparted to the radiation beam B by patterningdevice MA onto a target portion C (e.g. comprising one or more dies) ofthe substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem.

The term “patterning device” should be broadly interpreted as referringto any device that can be used to impart a radiation beam with a patternin its cross-section such as to create a pattern in a target portion ofthe substrate. The pattern imparted to the radiation beam may correspondto a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The projection system, like the illumination system, may include varioustypes of optical components, such as refractive, reflective, magnetic,electromagnetic, electrostatic or other types of optical components, orany combination thereof, as appropriate for the exposure radiation beingused, or for other factors such as the use of a vacuum. It may bedesired to use a vacuum for EUV radiation since other gases may absorbtoo much radiation. A vacuum environment may therefore be provided tothe whole beam path with the aid of a vacuum wall and vacuum pumps.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives an extreme ultra violetradiation beam from the source module SO. Methods to produce EUV lightinclude, but are not necessarily limited to, converting a material intoa plasma state that has at least one element, e.g., xenon, lithium ortin, with one or more emission lines in the EUV range. In one suchmethod, often termed laser produced plasma (“LPP”) the required plasmacan be produced by irradiating a fuel, such as a droplet, stream orcluster of material having the required line-emitting element, with alaser beam. The source module SO may be part of an EUV radiation systemincluding a laser, not shown in FIG. 1, for providing the laser beamexciting the fuel. The resulting plasma emits output radiation, e.g.,EUV radiation, which is collected using a radiation collector, disposedin the source module. The laser and the source module may be separateentities, for example when a CO₂ laser is used to provide the laser beamfor fuel excitation.

In such cases, the laser is not considered to form part of thelithographic apparatus and the radiation beam is passed from the laserto the source module with the aid of a beam delivery system comprising,for example, suitable directing mirrors and/or a beam expander. In othercases the source may be an integral part of the source module, forexample when the source is a discharge produced plasma EUV generator,often termed as a DPP source.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as facetted field and pupilmirror devices. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. After being reflected from thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor PS2 (e.g. an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor PS1 can be used to accurately position the patterningdevice (e.g. mask) MA with respect to the path of the radiation beam B.Patterning device (e.g. mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

An EUV membrane, for example an EUV pellicle PE, is provided to preventcontamination of the patterning device from particles within the system.Such pellicles may be provided at the location shown and/or at otherlocations. A further EUV membrane SPF may be provided as a spectralpurity filter, operable to filter out unwanted radiation wavelengths(for example DUV). Such unwanted wavelengths can affect the photoresiston wafer W in an undesirable manner. The SPF may also optionally helpprevent contamination of the projection optics within projection systemPS from particles released during outgassing (or alternatively apellicle may be provided in place of the SPF to do this). Either ofthese EUV membranes may comprise any of the EUV membranes disclosedherein.

FIG. 2 shows an embodiment of the lithographic apparatus in more detail,including a radiation system 42, the illumination system IL, and theprojection system PS. The radiation system 42 as shown in FIG. 2 is ofthe type that uses a laser-produced plasma as a radiation source. EUVradiation may be produced by a gas or vapor, for example Xe gas, Livapor or Sn vapor in which a very hot plasma is created to emitradiation in the EUV range of the electromagnetic spectrum. The very hotplasma is created by causing an at least partially ionized plasma by,for example, optical excitation using CO₂ laser light. Partial pressuresof, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas orvapor may be required for efficient generation of the radiation. In anembodiment, Sn is used to create the plasma in order to emit theradiation in the EUV range.

The radiation system 42 embodies the function of source SO in theapparatus of FIG. 1. Radiation system 42 comprises a source chamber 47,in this embodiment not only substantially enclosing a source of EUVradiation, but also collector 50 which, in the example of FIG. 2, is anormal-incidence collector, for instance a multilayer mirror.

As part of an LPP radiation source, a laser system 61 is constructed andarranged to provide a laser beam 63 which is delivered by a beamdelivering system 65 through an aperture 67 provided in the collector50. Also, the radiation system includes a target material 69, such as Snor Xe, which is supplied by target material supply 71. The beamdelivering system 65, in this embodiment, is arranged to establish abeam path focused substantially upon a desired plasma formation position73.

In operation, the target material 69, which may also be referred to asfuel, is supplied by the target material supply 71 in the form ofdroplets. A trap 72 is provided on the opposite side of the sourcechamber 47, to capture fuel that is not, for whatever reason, turnedinto plasma. When such a droplet of the target material 69 reaches theplasma formation position 73, the laser beam 63 impinges on the dropletand an EUV radiation-emitting plasma forms inside the source chamber 47.In the case of a pulsed laser, this involves timing the pulse of laserradiation to coincide with the passage of the droplet through theposition 73. As mentioned, the fuel may be for example xenon (Xe), tin(Sn) or lithium (Li). These create a highly ionized plasma with electrontemperatures of several 10⁵ K. Higher energy EUV radiation may begenerated with other fuel materials, for example Tb and Gd. Theenergetic radiation generated during de-excitation and recombination ofthese ions includes the wanted EUV which is emitted from the plasma atposition 73. The plasma formation position 73 and the aperture 52 arelocated at first and second focal points of collector 50, respectivelyand the EUV radiation is focused by the normal-incidence collectormirror 50 onto the intermediate focus point IF.

The beam of radiation emanating from the source chamber 47 traverses theillumination system IL via so-called normal incidence reflectors 53, 54,as indicated in FIG. 2 by the radiation beam 56. The normal incidencereflectors direct the beam 56, via pellicle PE, onto a patterning device(e.g. reticle or mask) positioned on a support (e.g. reticle or masktable) MT. A patterned beam 57 is formed, which is imaged by projectionsystem PS via reflective elements 58, 59 onto a substrate carried bywafer stage or substrate table WT. More elements than shown maygenerally be present in illumination system IL and projection system PS.For example there may be one, two, three, four or even more reflectiveelements present than the two elements 58 and 59 shown in FIG. 2.Radiation collectors similar to radiation collector 50 are known fromthe prior art.

As the skilled reader will know, reference axes X, Y and Z may bedefined for measuring and describing the geometry and behavior of theapparatus, its various components, and the radiation beams 55, 56, 57.At each part of the apparatus, a local reference frame of X, Y and Zaxes may be defined. The Z axis broadly coincides with the direction ofoptical axis O at a given point in the system, and is generally normalto the plane of a patterning device (reticle) MA and normal to the planeof substrate W. In the source module (apparatus) 42, the X axiscoincides broadly with the direction of fuel stream (69, describedbelow), while the Y axis is orthogonal to that, pointing out of the pageas indicated. On the other hand, in the vicinity of the supportstructure MT that holds the reticle MA, the X axis is generallytransverse to a scanning direction aligned with the Y axis. Forconvenience, in this area of the schematic diagram FIG. 2, the X axispoints out of the page, again as marked. These designations areconventional in the art and will be adopted herein for convenience. Inprinciple, any reference frame can be chosen to describe the apparatusand its behavior.

In addition to the wanted EUV radiation, the plasma produces otherwavelengths of radiation, for example in the visible, UV and DUV range.There is also IR (infrared) radiation present from the laser beam 63.The non-EUV wavelengths are not wanted in the illumination system IL andprojection system PS and various measures may be deployed to block thenon-EUV radiation. As schematically depicted in FIG. 2, an EUV membranefilter in the form of a spectral purity filter SPF (i.e. an SPFmembrane) may be applied upstream of the virtual source point IF, forIR, DUV and/or other unwanted wavelengths. In the specific example shownin FIG. 2, two spectral purity filters are depicted, one within thesource chamber 47 and one at the output of the projection system PS. Inone embodiment only one spectral purity filter SPF membrane is provided,which may be in either of these locations or elsewhere between theplasma formation position 73 and wafer W, such as at the reticle level.

Large DUV suppression may however be difficult at reticle level, sinceat that location back-reflection of out-of-band radiation is undesired(since it can affect the reticle shape). Therefore at reticle level thepreferred mechanism to suppress DUV and IR with a EUV membrane (e.g. EUVpellicle) is absorption only.

In another embodiment a first EUV membrane may be used at the reticlelevel to suppress particle debris depositing on the reticle and a secondEUV membrane may be used as a SPF membrane at the output of theprojection system PS (i.e. between the wafer and the last mirror of theprojection system PS). The SPF membrane is a EUV membrane operated as aspectral filter for blocking unwanted wavelengths of radiation. The SPFmembrane may be added in order to suppress out of band IR and DUVradiation, since near the wafer both reflection and absorption may beused to suppress the unwanted radiation.

Disclosed is an EUV membrane for transmission of EUV radiation, havingimproved thermal characteristics compared to present EUV membranes. SuchEUV membranes may comprise, for example (poly-)Si EUV membranes. Themembranes may be comprised within a spectral purity filter (SPF) or apellicle. SPFs and/or pellicles may be provided at many locations withina lithographic system, as already described.

In absorbing radiation during use, the EUV membranes heat up. Shouldtheir temperature increase too high or the temperature gradients withinthe membrane be too great, the EUV membranes can be damaged. Thereforeit is desirable to minimize temperature and temperature gradients withinthe EUV membrane. As the EUV membranes will be used in very low pressure(vacuum) environments, the only means of cooling is radiation. It istherefore desirable to increase thermal emissivity ((i.e. improve IRemissivity) of the EUV membrane for the wavelengths (for example 1 to 10μm) at which most energy is radiated when the temperature of the EUVmembrane ranges from about 100 to about 1000° C., more preferably fromfew hundred (e.g. at least 200° C.) to about 1000° C., and morespecifically at moderate temperatures (less than 500° C., such as from100 to 500° C.). In these conditions for example a pure (i.e. bulk)layer of (poly-)silicon material presents a low thermal emissivity,since all free charge carriers are still bound.

Simulations based on multilayer Fresnel reflection coefficients andPlanck's law for calculating the hemispherical infrared absorption(which relates to emissivity) have been done to understand the change inIR absorption (thermal emissivity) of thin membranes as a function oftheir thickness. Such simulations have shown that films of dielectricmaterials such as SiC and Si will become less IR radiation absorptive asthey get thinner. Therefore, EUV membranes from dielectric materials(which are required to be thin to provide a substantial EUVtransmission) will generally have little IR absorption/emission on theirown.

To increase emissivity in an EUV membrane comprising a semiconductormaterial, the EUV membrane material may be doped to increase the numberof free charge carriers within the material. This increases theradiation absorption coefficient of the doped membrane, which leads toan increase in the emissivity. The skilled reader will know that dopingof semiconductor materials with donors and/or acceptors modifies thefree charge carrier concentration (electrons and/or holes) at moderatetemperatures.

The concentration of impurity to be doped into the semiconductormembrane should be higher than 10¹⁷ cm⁻³ for a significant effect.Concentrations may preferably be higher than 10¹⁸ cm⁻³, 10¹⁹ cm⁻³ or10²⁰ cm⁻³. It can be shown that absorption coefficients can increase bya factor of 1000 at radiation wavelengths greater than 1.2 μm when thedopant concentration is increased from 10¹⁷ cm⁻³ to 10²⁰ cm⁻³. Thisapplies equally to doping with p-dopants and n-dopants.

However, adding dopants tends to reduce the strength of semiconductormaterial such as polysilicon. This is particularly a problem from EUVmembranes due to their need to be particularly thin in order to transmitthe EUV radiation with the minimum amount of loss. Consequently a numberof solutions are proposed to address this.

FIG. 3 is a schematic diagram of a EUV membrane 300 which is positionedin front of the patterned area of a reticle MA. EUV membrane 300 isshown here as forming part of a pellicle designed to keep particles Doff the patterned area of reticle MA, while allowing transmission of EUVradiation beam 305. In such an example EUV membrane 300 may comprise anEUV membrane within a pellicle frame (not shown). The EUV membrane 300may further comprise (for example) securing elements for attaching thepellicle frame to the reticle (not shown). EUV membrane 300 may beplaced out of the focal plane, at some distance from reticle MA, suchthat contaminants are not imaged onto the wafer.

In other embodiments, EUV membrane may form part of a pellicle for usein another location within a lithographic apparatus, or an SPF.

The EUV membrane 300 may comprise a number of layers. These layers mayinclude the main substrate layer 310, cover layers 311, 312, andintermediate layers 313, 314 which may be for example anti-diffusionlayers 313, 314. The main substrate layer 310 may be, for example, a(poly-)Si layer. This arrangement is shown by way of example only, andother combinations of the layers shown are possible. For example, theEUV membrane 300 may comprise cover layers 311, 312 without anyintermediate layers. In another exemplary alternative, there may be onlyone cover layer on just one surface of the main substrate layer (with orwithout an intermediate layer between cover layer and substrate layer).There could also be more than two layers on one or both surfaces of themain substrate layer.

Typically, cover layers 311, 312 are made of a (inert) material toresist any etching or reacting agents that can harm the main substratelayer 310, e.g., O and H radicals, H₂ and EUV. Examples of such amaterial include MoSi₂, Si₃N₄, C₃N₄, ZrN, SiC. Such materials typicallyhave a wide forbidden energy zone and are similar in properties toceramics. Consequently, such materials have high emissivity even atmoderate temperatures, for example less than 500° C. Moreover thesematerials are produced from elements with low absorption of EUV, whichis comparable with pure Si absorption. Therefore, provided that thecover layers 311, 312 have a much smaller thickness than main substratelayer 310, they do not significantly increase overall EUV absorption ofEUV membrane 300. The cover layers 311, 312 should also not place toogreat a stress on the main substrate layer 310, so as to preserve itsmechanical properties.

Intermediate layers 313, 314 may be provided to reduce the stress. Forexample intermediate layers 313, 314 may comprise material having anintermediate lattice size between the main substrate layer 310 and coverlayer 311, 312. Intermediate layers 313, 314, like the cover layers 311,312, should be highly transparent to EUV.

In an embodiment, the covers layers 311, 312, and/or the intermediatelayers 313, 314 (if present) may be doped to increase the concentrationof free charge carriers, as already described. In this way the coverslayers 311, 312, and/or the intermediate layers 313, 314 form high dopedregions within the membrane. The main substrate layer 310 may be formedas a low doped region to maintain strength. The doping of one or more ofthe other layers 311, 312, 313, 314 significantly increases emissivityof the EUV membrane 300 as already described.

High doped regions have a dopant concentration of at least 10¹⁷ cm⁻³,while low doped regions have a dopant concentration less than 10¹⁷ cm⁻³.Doping levels of the high doped regions may be any of those describedabove, in relation to the doping of the semiconductor membrane, and assuch may be higher than 10¹⁸ cm⁻³, higher than 10¹⁹ cm⁻³ or higher than10²⁰ cm⁻³, for example. Doping levels of low doped regions, such as themain substrate layer (i.e. the core layer), may be less than 10¹⁶ cm⁻³,less than 10¹⁶ cm⁻³, or less than 10¹⁴ cm⁻³, for example. Low dopedregions may be undoped and therefore have no (intentional) addeddopants.

FIG. 4 shows an alternative embodiment showing EUV membrane 400 havingthe same layer structure as EUV membrane 300, but also comprisingadditional cover layers 411, 412 placed on cover layers 311, 312, asshown in FIG. 4. These additional cover layers 411, 412 may be highdoped regions instead of (or in addition to) the cover layers 311, 312.The doping concentrations of the additional cover layers 411, 412 may beany of those mentioned in the previous paragraph.

By doping only the cover layers 311, 312, 411, 412 or intermediatelayers 313, 314, rather than the main substrate layer 310, the weakeningeffects of the doping are mitigated and the overall EUV membrane 300 isstronger as a result.

FIG. 5 illustrates another embodiment. It shows an EUV membrane 500,which may comprise only a single main substrate layer, or alternativelymay comprise cover/intermediate layers, such as layers 311, 312, 313,314 and possibly also layers 411, 412. In this embodiment, one or moreof: the main substrate layer, and (where present) the cover/intermediatelayers comprise doping (which may be at the concentrations alreadydescribed), but where the high doped regions are limited to only acentral region 510 of the layer doped. The periphery 520 of this dopedlayer is a low doped region, where it may be held by a frame. Thisincreases the strength of the EUV membrane 500 at its periphery, whichis subject to greater stresses due to holding by the frame. It should beappreciated that the peripheral area 520 transmits little or no EUV, asthis is mostly or completely transmitted through the central region 510.Consequently the peripheral area 520 is subject to little heating andits thermal characteristics are less important.

Optionally, the doping can be graded, such that doping increases towardsthe center. In such arrangements, the gradient may occur over the fullradius of the EUV membrane, or layer thereof (i.e. doping starts at themembrane edge and increases towards the center). Alternatively dopingmay only begin at the edge of the central region 510 and increasetowards the center, with the peripheral region 520 having no doping. Orthe doping grading may occur for only an intermediate section between aperipheral region having no doping and a central region having highdoping.

Using a similar principle to that described in the previous paragraph,doping can be introduced to any layer in the form of spot doping. Spotdoping comprises a plurality of high doped (high emissivity) regions,separated by regions of no or low doping (and therefore greaterstrength). Again, this concept can apply to an EUV membrane 500,comprising only a single main substrate layer, or to EUV membrane 500comprising additional layers, such as cover layers and/or intermediatelayers, in which case the doping can be introduced to any one or more ofthese layers. In an example, the high doped regions may be separatedfrom one another by approximately 1 μm to 5μm. It should be appreciatedthat the heat flux to the highly doped regions is by phonons withcomparable or even longer wavelengths than this. Heat is transferred bytwo mechanisms: radiation (photons) and heat conduction (oscillation ofatoms within the lattice, phonons). When the distance between wherepower is deposited (undoped region) and where power is removed (highdoped region) is close, the power is transferred significantly faster;close may be defined as being comparable to wavelength of a phonon witha typical energy (defined by temperature, such a wavelength is in theregion of a few microns).

Of course the concepts described in the previous paragraphs may becombined such that the spot doping is confined only to a central region510 of an EUV membrane, or layer thereof, with no doping in theperipheral region 520. And the doping concentration may be graded suchthat high doped regions nearer the periphery are less highly doped thanthose nearer the center. This can help control thermally induced stressand the cooling rate (both of which are a function of dopantconcentration). This can also help to control deformations such aswrinkles or folds being formed. When the temperature of the EUV membraneis increased, the material of which it is comprised expands. The flatplane, which is the nominal shape of an EUV membrane, cannot accommodatethe expanded material, and folds or wrinkles are formed. EUV radiationabsorption by the folds is higher as EUV radiation crosses the EUVmembrane at an angle, and thus the effective absorption path is longer.The folds may have a transverse scale of about 10 micrometers or larger(across) and will be imaged on the wafer. Using spot doping, the typicalscale of the folds is defined by the geometry and scale of high dopedand low doped regions due to the combined effect of temperature profilecontrol and mechanical properties control. Where the temperatureincreases, the angles of the folds in a spot-doped membrane are thesame, but the transverse size is decreased and therefore such folds areno longer imaged.

Previous studies have shown that, for example, photon tunneling andsurface polaritons may play a key role in near-field radiative energytransfer when separating distances between radiating objects are smallerthan dominant thermal wavelengths. For example, a study by B. Liu etal., Phys. Rev. B 87, 115403, (2013), has demonstrated that near-fieldradiative heat transfer of some materials can exceed the blackbodyradiation limit by few orders of magnitude due to energy transferthrough evanescent waves. The studied material supported surfacepolaritons in the IR region (for example, doped Si materials, SiC, BN orany suitable material that might be used as candidate materials forcover layers 510 and 514).

A graph comparing a near-field radiative heat transfer between twosemi-infinite plates made of SiC and gold as function of distance d canalso be found in B. Liu et al. (FIG. 1). Distance d represents thevacuum gap size between the two plates. As can be seen in FIG. 1 of B.Liu et al., near-field radiative heat transfer between plates made ofSiC and gold is three orders of magnitude less than the heat transferbetween two SiC plates.

Consequently, in order to further improve transverse radiative heattransfer along pellicles, in an embodiment it is proposed to provide aplurality of additional features on one of the EUV membrane surfaces.These additional features can be grown or formed during the etchingprocess. The additional features may be of any suitable shape. In oneexample the additional features comprise periodic or aperiodic wires orthin walls or ribs extending normal from the EUV membrane surface. Theadditional features may comprise doped Si or Si-based materials or anysuitable cover layer material, such as any of the materials, having anyof the dopant concentrations and arrangements disclosed herein. Thefeature size of each additional feature should be significantly smallerthan the size of the area bounded by the features. It can be shown that,if the distance between additional features is ≤1 μm, the radiative heattransfer is expected to be 10-10000 times higher than the blackbodylimit.

FIG. 6 shows an EUV membrane 600 comprising a plurality of additionalfeatures 620 (e.g. formed by periodic or aperiodic wall or wirestructures 620). The additional features 620 may be located on the lowerside of the EUV membrane 600 (the side exposed to EUV radiation). Theside of the EUV membrane facing the reticle may be flat to maintainpurity. Radiative heat transfer is symbolized by vertical arrows 630.Horizontal arrows 640 symbolize a transverse radiative heat transfergenerated by the additional features 620. Note that illuminating EUVradiation (not shown) propagates almost normal to pellicle P. Therefore,the additional features 620 (in the form shown here, i.e., wires orribs) cast a minimal shadow on reticle MA and/or wafer W.

Transverse temperature gradients in the EUV membrane are believed tocause as much damage to the membrane as high temperatures by themselves.While all the embodiments described herein significantly reducetemperature gradients in the EUV membrane during exposure to EUVradiation, the embodiment depicted in FIG. 6 is particularly effectivesince transverse heat conduction is increased compared to a flatmembrane case (where temperature is only transferred by phonons) byadding another mechanism: radiation heat transfer. It is believed thatheat transfer from EUV membrane to an additional feature 620 is notlimiting, since the typical scale is small. An efficient transverse heattransfer would minimize these temperature gradients and extend lifetimeof the pellicle.

FIG. 7 is a graph of expected temperature distribution against distanceL across the EUV membrane. Line P_(EUV) represents the EUV radiationpower distribution across the pellicle. Line T_(A) represents thetemperature distribution of a flat EUV membrane. Line T_(B) representsthe temperature distribution across the EUV membrane depicted in FIG. 6.As can be seen from FIG. 7, temperature gradients across the EUVmembrane are reduced for the FIG. 6 example, compared to a flat EUVmembrane.

FIG. 8 shows a further embodiment of an EUV membrane 800, comprising arefinement to the embodiment depicted in FIG. 6. In this embodiment, theadditional features 820 comprise a shape and/or formation which mimicsthat of an echelette grating. In the specific example, the additionalfeatures comprise repeated groups of wires or ribs 820, with theindividual wires/ribs 820 of each group descending (or increasing)progressively in height as shown. The result is an approximation of anechelette grating, which is illustrated by the dotted line. An echelettegrating-like structure helps to direct unwanted radiation 830,originating from scattering of EUV radiation by each wire/rib 820individually, away from orders (e.g. 0 and 1^(st) orders) of the EUVradiation 840 during transfer of a pattern from reticle MA to the wafer.

FIG. 9 illustrates the emissivity of doped EUV polysilicon pellicle of60 nm thickness (left side graph in FIG. 9) and the integratedemissivity versus temperature in K for intrinsic polysilicon pellicle vsdoped pellicles (right side graph in FIG. 9). To increase the emissivityabove 0.1 a 60 nm polysilicon pellicle was doped with at least 5×10¹⁹cm⁻³.

In all the above embodiments, doping materials may be limited to thosetransparent for EUV, and which have the smallest mismatch with Silattice (e.g. carbon, boron and nitrogen) for the sake of strength andreliability. In other embodiments, dopants which are not transparent for13.5 nm but are transparent to other EUV/DUV wavelengths can be used,where the wavelength is appropriate for the lithographic system. Thesedopant materials may include: S, Te, As, O, Al, Sn, Sb, In, Ga, Br, Cl,I, C, B, N.

Although polysilicon has been taken above as main example of an EUVpellicle core layer material (since it is the most transparent materialat 13.5 nm EUV radiation), doping of an EUV pellicle material withimpurities in order to increased emissivity may be done for anysemiconductor. Doping may be done using B or P, which are bothtransparent materials in the EUV regime. If silicon is doped with B or Palso the EUV loss is negligible.

In order to increase an EUV pellicle emissivity to IR radiation whileEUV transmission is still substantial, it is herein alternatively or inaddition to doping also proposed to coat the EUV pellicle with a caplayer for improved IR emissivity comprising a material which is goodabsorber for IR radiation but transparent in the EUV radiation regime,for example with a metal cap layer. Such a cap layer may in additionprotect the pellicle from oxidation or other environmental hazards.

The metal cap layer should be a closed film, i.e. metal islands aregenerally not preferred since the resistivity can go up a factor of10,000 and the Drude absorption term is canceled. Such inhomogeneousfilms could become transparent and thus provide insufficient absorption.

The EUV pellicle may be chosen to be transmissive for 90% or more of agiven EUV radiation wavelength, such as 13.5 nm or 6.8 nm (or any otherEUV radiation wavelength). As an example, a polysilicon pellicle of 45nm thickness coated on both sides with 3 nm Si₃N₄ has about 85% EUVtransmittance, will have a poor (almost no) IR emissivity (i.e. it mayget very hot), it reflects much of DUV radiation present in theradiation spectrum (which is not desired for imaging purposes) and willhardly transmit any DUV radiation at all (which results in no option forperforming through pellicle inspection to detect particulate debris).

In an aspect of the invention there is provided a membrane transmissiveto EUV radiation (i.e. an EUV pellicle) having a core layer materialselected from (poly-) Si, Si₃N₄, SiC, ZrN, ZrB₂, ZrC, MoB₂, MoC, RuB₂,LaB₂, LaC, TiB₂, TiC, (poly-) crystalline Yttrium, (poly-)crystallineZr, Be, C, B and B₄C and composites or combinations of multilayerstherefrom. Semi-metals such as ZrB₂, ZrC may reduce the electrostaticcharging of the EUV pellicle. Silicon nitride Si₃N₄ (also referred toSiNx) refers herein to amorphous silicon nitride and incorporates bothstoichiometric (3:4 ratio, x=1.33) and non-stoichiometric SiNx alloys(0<x<1.6).

The EUV pellicle has preferably a thickness of 60 nm or less to allowsufficient EUV transmission (preferably at least 90% EUV radiationtransmission). In order to provide sufficient strength to the EUVmembrane it may be needed that the core layer has a minimum thickness ofat least 5 nm, preferably at least 10 nm and more preferably at least 15nm minimum thickness.

The EUV pellicle core layer (also referred to as main substrate layer)may be capped on one or both sides with a metal layer or another caplayer (also referred to as cover layer) from a material having athickness suitable to increase IR emissivity. Examples of suitable caplayer metals that have good EUV transmittance are Ru, Ti, Nd, Pr, Mo,Nb, La, Zr, B, Y and Be. These and other metals may also be used in asimilar way to coat the EUV pellicle (more specifically the core layer)and can provide improved IR emissivity. For example a pellicle having aB or Be core layer and being capped with a layer of Ru, Mo or othermetal (composite) cap layers may provide a substantially improved IRemissivity.

Metallic thin layers have an emissivity strongly affected by plasmafrequency. Metals such as Ru which are less conductive have less freecharge carriers and thus lower plasma frequency, being therefore betterchoice for improved IR emissivity than more conductive metals such as Auor Ag. The highest value for plasma frequency is around 10 eV for Al. Aufilms have plasma frequency varying from 7 to 9 eV depending on the filmquality.

The cap layer may also be a composite material comprising a metal andEUV transparent impurities. By adding non-metallic or poorly conductiveEUV transparent impurities the plasma frequency may be tuned to lowervalues, in which case many metals become good candidates as EUV pelliclecap layers with improved IR emissivity. Examples of poorly conductiveimpurities are boron, nitrides, carbon, silicon, strontium calcium andphosphorus. By adding impurities and lowering the plasma wavelength, themetal layer thickness may be increased. The impurity concentration insuch case is preferably less than 10% atomic percent.

To protect the metal cap layer in the pellicle membrane stack (i.e. inthe multilayer stack comprising one or more core (multi-)layers and atleast one cap layer for improved IR emissivity), an additionalprotective cap layer may be included on top of the metal cap layer forimproved IR emissivity. This protective cap layer may diminish theeffects of oxidation and etching in the EUV plasma environment. Examplesof materials suitable for such a protective cap layer may be oxides,carbides or nitrides of the following materials: Zr, Ti, Hf, Si, Rh orRu (e.g. ZrO₂, ZrN, ZrC, etc.). The thickness of these protective caplayers is preferably in the order of 1 to 3 nm.

It has been found that metal layers which normally reflect IR radiationbecome more absorptive when their thickness is less than the skin depth.Metal layers as thin as 1 nm may have a nearly flat spectral responseand emissivity close to the theoretical limit of 0.5. A reason forincrease in absorption with the decrease of layer thickness may be thelarge absorption coefficient for metals and reflection canceling due todestructive interference at the metal-vacuum and metal-dielectricinterfaces.

In an aspect of the invention there is provided a membrane transmissiveto EUV radiation, which is coated with a cap layer for improved IRemissivity comprising a metal cap layer of thickness<the skin depth of ametal in IR radiation. The skin depth thickness of the metal cap layerfor IR radiation may generally be <10 nm, although there are metals suchas Yttrium (Y) which could still work according to the invention with athickness a bit larger than 10 nm. Skin depth means herein the thicknesswhere light has lost 63% of its intensity (or has intensity 1/e). Theskin depth depends on the light wavelength. Most metals have generally askin depth of around 10 nm in IR radiation (i.e. IR radiationpenetrating a 10 nm metal layer will lose 63% of its intensity).

Thin metal cap layers basically act as IR absorbers, whereas thetransmission of the EUV radiation may be substantially the same. Forexample, it has been determined that a (poly-)silicon pellicle having acore of 58 nm thickness and a layer of 1 nm Ru on each side of thepellicle (since Ru has good EUV transparency), it has for a 13.5 nm EUVradiation a transmittance of 0.878, as compared to the transmittance of0.9 for a (poly-)silicon pellicle of 60 nm thickness. However, when theEUV pellicle is coated on one or each side for example with 1 to 2 nm Rucap layer, the emissivity of a (poly-)silicon pellicle may go up by afactor of 10 or more. Ru or other metals on (poly-)Si membranes mayenhance a EUV membrane emissivity from <0.01 up to 0.4 or more. Howevercare should be taken, since Ru or Mo with a thickness close to ¼^(th)wavelength of the EUV radiation it may reflect around 1% of EUVradiation, which may be detrimental to CD uniformity. Calculations haveshown that Ru cap layers with thickness close to 1 nm on EUV membranesmay have a reduced EUV reflection and still have some IR emissivity.Also Ru with half wavelength thickness (e.g. 6.7 nm thickness for 13.5nm EUV radiation) may act as an anti-reflective (AR) coating (with noEUV reflection); however when the Ru cap layer thickness was around1/4^(th) EUV wavelength in such case the EUV reflection had the highestvalue.

As a strategy to reduce EUV reflection (EUVR) of an EUV membrane coatedwith a single metal cap layer for improved IR emissivity or any otherfunction, it is proposed herein that the thickness of the metal layer Dis a multiple of half wavelength λ of the EUV radiation used forlithographic exposure (e.g. 13.5 nm, 6.7 nm or 4.37 nm EUV radiation):

D=nλ/2

with n being an integer=3, 4, 5, 6 or more. Preferably n has a valuesuch that the metal cap layer has a thickness smaller than the metalskin depth in IR radiation.

Other AR strategies for low EUVR may be to take a low metal cap layerthickness of 2 nm or less, such as between 1 and 2 nm (i.e. make the IRemissivity enhancing cap layer thin enough so EUV reflection is lower),or to have rough, non-sharp diffuse boundaries.

In the case of even number of metal cap layers for improved emissivity,such as two metal cap layers, the reflectivity of the individual metallayers follows the same rules as for one metal layer. It is hereinproposed an EUV membrane in an anti-reflection configuration wherein thetwo metal cap layers are separated by another core layer of thicknessaround half of the EUV radiation wavelength λ/2 used for lithographicexposure, such that destructive interference of EUV radiation occurscanceling each other and thereby the net (resulting) EUV reflection iszero.

For example, if two layers of 2 nm Ru or Mo are separeted by a (poly-)Silayer with a thicknesses selected from 8.4 nm, 15.1 nm, 21.9 nm, 28.6nm, 35.4 nm, 41.5 nm, 48.7 nm and 55.7 nm (i.e. in steps of roughly 6.7nm), in such case the second Ru cap layer induced reflection interferesdestructively with the reflection of the first Ru cap layer and therewill be no EUV radiation reflection. It is mentioned that the thicknessof the polysilicon core layer is not exactly half of the EUV radiationwavelength of 4.37 nm, or 6.7 nm, or 13.5 nm, as it may also beinfluenced by the thickness of the metal cap layer. Therefore thegeneral condition for layer thickness in order to avoid EUV reflectionfor any combination of core layer covered with one or more pairs ofmetal cap layers is such that completely destructive interference occursbetween the metal layers such that no EUV radiation is reflected.

In all the above alternatives for reduced EUV reflection high emissivitycan still be maintained due to the metal cap layer, while EUV reflectionis minimized (i.e. impact on imaging is minimized), enabling EUVpellicles with high IR emissivity while maintaining low EUVreflectivity.

By itself, even just a core layer of 50 nm (poly-)Si can alreadysuppress DUV radiation by a factor of 100 or more. (Poly-)Si has almostno transmission in the range of 100-400 nm where DUV radiation isexpected. (Poly-)Si pellicles are transparent however in the IRradiation range. It has been found that IR transmission through the 50nm (poly-)Si core layer can be suppressed by a factor of 20, by adding ametal cap layer such as Ru or Mo to the core layer. Furthermore it maybe advantageous to use anti-diffusion barrier layers (such as from B₄Cor SiNx) for the metal cap layers such that the metalic reflection andabsorption is not lost due to diffusing into the core layer (e.g. Ru orMo diffusing in (poly-)Si).

Although a given material may be suitable for multiple purposes, such asfor a core layer, a cap layer or even an anti-diffusion barrier layer,the layer thickness and position in the EUV membrane may provide usefulcriteria to define the function of such a layer. The thickness of aninterdiffusion layer for instance is generally 1 nm or less.

For example, a layer of B or B₄C having the thickness of 1 nm or lessand being located between the core layer and an adjacent cap layer mayserve as anti-diffusion layer, while a layer of the same materialshaving a thickness of 4 to 11 nm may serve as a core layer if itprovides high tensile strength in comparison with other layers. In asandwich-like configuration of 10 nm B—(5-10 nm) Mo—10 nm B for instancethe two B layers will form the core layers and Mo in between forms a caplayer for improved IR emissivity which is protected from etching.

In the same way, a layer of B or B₄C on the top of the EUV membrane orsandwiched between other (core) layers may serve as a cap layer with agiven function. Furthermore, a multilayer stack of thin layers which asa total stack has a high yield strength >50 MPa may also form a corelayer. For example, up to 20 pairs of layers of graphene (graphiticlayers) between boron, such as 10 nm B/3 nm graphene/10 nm B, mayprovide an advantageous multilayer EUV membrane since B is expected tobe chemically resistant under EUV and/or H₂ atmosphere and graphite willprovide improved emissivity and mechanical strength. Another example ofa multilayer EUV membrane comprises several (up to 20) layers ofgraphene (or graphitic type layers) on top of SiNx layer or othermembrane cap layers to provide mechanical strength, improve emissivity,and increase lifetime of the EUV membrane under EUV and/or H₂atmosphere. For example a multilayer EUV membrane comprising layers of 2nm graphene (i.e. multilayers or multiple sheets of graphene to achievea thickness of 2 nm)/10 nm SiNx/2 nm graphene may similarly form anadvantageous EUV membrane. The person skilled in the art knows how todifferentiate between core and cap layers.

In order for the IR suppression to work it doesn't matter in principlewhere the metal cap layer is deposited. It may be on top, bottom or inthe middle of an EUV membrane multilayer stack (such as a sandwichstructure).

Because (poly-)Si may etch in the EUV environment, as alternative EUVmembrane a sandwhich membrane structure of a molybdenum cap layerbetween two boron cap layers (B—Mo—B) is proposed above (since Ru is3×more absorptive for EUV radiation than Mo; and because Mo may oxidizewhen exposed to ambient). The combination of boron+metal may have equalIR suppression as (poly-)Si+metal, however the DUV suppression is lessthan for (poly-)Si (a factor 7+instead of a factor 100+).

EUV transparent metals are for instance Ru, Mo, La, Rh, Be, Y, Zr, Ce,Nb and Pr. Capping layers of boron, B₄C, Si₃N₄, ZrO₂, Ru or MoSi₂ orother alternative cappings may be advantageous for (poly-)Si SPFmembranes.

A metal thicknesses of at least 1 nm, in some conditions in excess of 5nm may be required for good IR absorption. Too thin metals will haveoptical response quite different from bulk. IR reflection by metals cantherefore be greatly diminished if the metal layer becomes too thin.

In general, any thin metal cap layer with thickness <skin depth of thatmetal in IR radiation is suitable for an EUV membrane with improved IRemissivity according to the invention. However if the EUV membrane isused as a SPF membrane then it is advantageous if the metal cap layerthickness >5 nm such that it is also quite reflective, such that themetal cap layer applied on a IR tranmissive core layer will reduce theIR tranmission by an order of magnitude or more. Although thedisadvantage of a thicker metal cap layer is more EUV radiation loss (upto 10-15%), there is still a substantial gain in terms of filtering IRand DUV radiation (e.g. 100× or more DUV 100-400 nm suppression and 20×IR (10.6 microns) suppression for (poly-)Si or B core layer with a 5-10nm metal cap layer.

If an EUV membrane is used as a SPF membrane located between theprojection system PS and the wafer, it may also be advantageous to havea membrane configuration oriented under a small angle in the scandirection of the lithographic apparatus, such that the reflectedout-of-band, IR and DUV radiation are not reflected back into theprojection system PS of the lithographic apparatus. Also, an absorptionscreen may be needed on one or more of the EUV mirrors of the projectionsystem PS in order to protect them from the additional DUV and IRabsorption and back reflection.

The thickness of the cap layer for optimal IR absorption (i.e. improvedIR emissivity) may be in a different range than exemplified above forRu, depending on the cap layer material. However, for allowing asubstantive EUV transmission it is generally advantageous to keep thethickness of the cap layers as small as possible. The thickness of allthe cap layer(s) stacked on the EUV pellicle should preferably be 90 nmor less, preferably 50 nm or less, more preferably 20 nm or less, evenmore preferably 10 nm or less (about the metal skin depth in IRradiation) and most preferably 5 nm or less , depending on the choice ofmaterials.

Table 1 shows examples of the thickness (in nm) of the above listedmaterials for a cap layer with improved IR emissivity, for which layerthickness the theoretical 13.5 nm EUV transmissivity is about 90%.

If two cap layers for improved IR emissivity are used (e.g. one for eachside of the EUV pellicle) then the thickness of each cap layer may betaken as half of the optimal cap layer thickness in order to still keepa good EUV transmission. In similar way, if several (three or more) caplayers for improved IR emissivity are used the individual and totalthickness of the cap layers will have to be adjusted such that a goodEUV transmission is still kept Above is referred to cap layers, howeverthe materials listed in table 1 may also form the core layer of the EUVpellicle, as long as suitable mechanical strength can be achieved tomanufacture a self-standing pellicle.

Also oxidation is a concern for many of these cap layer materials. Usingnitrides (for example ZrN (13 nm) or LaN (10 nm)) may help againstoxidation although a nitride may introduce more EUV loss. Ru coatinglayer(s) having each a thickness in a range from 0.5 to 5 nm, preferablyfrom 1 to 3 nm, more preferably from 1 to 2 nm is therefore one of thepreferred choices to improve the EUV pellicle emissivity in IRradiation.

Ru is given herein as an example because it has good anti-oxidationproperties (for a protective cap layer) and good EUV transmittance.Herein a new function is proposed for Ru as material for IR emissivityenhancing cap layer. IR emissivity enhancement may however be obtainedwith any metal cap layer (e.g. also gold or silver), but the EUVtransmittance may become worse. The inventors have found severalmaterials exemplified herein which are both substantially EUVtransparent and have a Drude behavior of electrical conduction (whereinelectrons act as free charge carriers bouncing and re-bouncing offheavier, relatively immobile positive ions).

Another example of an EUV pellicle is a carbon-based material forpellicle core having for example a thickness of 4 to 7 nm. Bycarbon-based material herein is meant any carbon structures in variousallotrope forms, also including carbon nanostructures in form of a ball,tube (cylinder) or sheet. Examples of carbon-based materials are carbonnanotubes, graphene, graphite, diamond-like carbon (DLC),(Buckminster-)fullerene or other C structures. Herein carbon-basedmaterials are for simplicity also referred to as carbon.

EUV pellicles having a core layer from a carbon-based material may alsofunction well for an EUV radiation of 4.37 nm wavelength. Such EUVpellicles may have relatively low IR emissivity. Coating the pelliclecore with thin metal cap layers such as Ru, Pd, Ag, Ti, Mo, Zr or Nblayers will not hinder EUV transmission much, but it will significantlyenhance the IR emissivity.

A pellicle having a SiNx core (11-12 nm) capped with a 2 nm Ru cap layergives about 90% transmission and can withstand high EUV power. Rucoating on both sides of the SiNx pellicle may lead to about 4%additional loss. Such membranes show a significant absorption in the VISand NIR range. For example, for heat load testing done with pulsed 90Watt (λ=355 nm) and 60 Watt (λ=810 nm) lasers on a 1 cm² area of a 13 nmthick Si₃N₄ membrane, covered on each side with a Ru layer of 2 nm,which membrane had around 85% EUV transmission, the heat load testingresults showed that such membrane could survive a heat load of 170 Wattsfor over 200,000 laser shots without significant change in the EUVtransmission (EUVT).

A pellicle having a B₄C or boron (B) core (20 nm) capped with 2 nm Rucap layer gives about 90% EUV transmission. A boron based EUV pellicle(core) has a self-limiting oxide (since oxide does not diffuse veryeasily in boron). Boron is also very etch-resistant and we can work alsowith only one layer of Ruthenium (Ru). Reversely, also a Ru layer as caplayer for improved IR emissivity may be embedded between two B corelayers.

Generally speaking, when IR emissivity is increased (in any way) frombelow 0.1 to about 0.5, the pellicle temperature can decrease from about800 degrees to 400 degrees Celsius. This will lower the heat inducedstresses in the pellicle core membrane and therefore increase lifetimeof the pellicle at higher EUV source powers. Advantages of such measuresmay be one or more of the following: at least 10× higheremissivity/radiative cooling for pellicles, much cooler pellicles duringexposure, and pellicles that survive higher heat loads (i.e. higher EUVsource powers).

FIG. 10 compares the EUV pellicle power absorption and maximumtemperature versus EUV source power. When a (poly-)Si membrane mightsurvive around 40 W source power, a (poly-)Si pellicle having a Rucoating for improved IR emissivity can enhance the power absorption to500 W source power such that the EUV pellicle remains intact. FIG. 10shows the absorbed power and equilibrium temperature (in ° C.) for EUVpellicles of 60 nm Si, 25 nm SiC, 12 nm Si₃N_(4, 40) nm Si+3 nm Ru, 19nm ZrB₂ and 20 nm ZrC.

FIG. 11 shows the equilibrium temperature vs. EUV radiation transmission(EUVT) and emissivity for 50 mj/cm² power equivalent to EUV source powerof 250 W. With a 250 Watts source and a pellicle with 90% transmissionmay absorb about 1 Wcm⁻² EUV radiation, which is re-emitted at theequilibrium temperature. Emissivity below 1% in the case of polysiliconfilms results in temperatures of over 1000° C. and pellicle failure. Rucoated pellicles with emissivity of 0.4 can reduce this temperature forexample to a more manageable temperature around 600° C.

Also a silicide cap layer may be effective in increasing IR emissivity,such as ZrSi₂ or NbSi₂ as IR-emitting cap layers. They may be coveredwith a protective cap layer of ZrO₂ and Nb₂O₅, respectively. Silicidesmay be even better than Ru with respect to transmission of EUVradiation. For example a combination of ZrSi₂/ZrO₂ cap layers may have ahigher transmission for EUV radiation than a stack of NbSi₂/Nb₂O₅ caplayers.

Examples of suitable materials for high temperature resistant pelliclesat 13.5 nm EUV radiation are ZrB₂, ZrC, MoB₂, MoC, RuB₂ and SiC.

Examples of suitable materials for high temperature resistant pelliclesat 6.7 nm EUV radiation are ZrB₂, ZrC, LaB₂, LaC, TiB₂, TiC MoB₂ andMoC. For 4.37 nm EUV radiation a suitable material is for example TiC.

If the cap layer for improved IR emissivity is located on the top of thecore layer, such that it comes in direct contact with external degradingfactors (e.g. H radicals, EUV radiation etc.), then relatively fastfailure of the cap layer/EUV pellicle could arise due to the highpellicle temperature during EUV exposure. In an embodiment it isproposed to sandwich a cap layer for improved IR emissivity between twochemically resistant core layers (such as between two boron, carbon orB₄C layers) to avoid degrading. The cap layer preferably is a metallayer. Examples of configurations (and suitable thickness ranges) tunedfor at least 90% transmission in a Boron or B₄C (5-10 nm)-metal (1-10nm)-Boron or B₄C (5-10 nm) configuration are:

-   -   Boron (B₄C) 11 nm-Mo 5 nm-Boron (B₄C) 11 nm;    -   Boron (B₄C) 11 nm-Y 10 nm-Boron (B₄C) 11 nm; and    -   Boron (B₄C) 10 nm-Ru 3 nm-Boron (B₄C) 10 nm.

When the core layer in the EUV pellicle sandwich structure is boron orB₄C, suitable metal cap layer materials for improved (enhanced) IRemissivity for EUV wavelength of 6.7 nm are for example Nb, Mo, La, Zr,In, Ti, Ru, Te, Bi, Ce, Pd, Ag and Y.

When the core layer in the sandwich structure is carbon or acarbon-based material (e.g. carbon-metal-carbon), then suitable metalcap layer materials for improved (enhanced) IR emissivity for EUVwavelength of 4.37 nm are for example Be, La, Te, Ti, Pr, Rh, Eu, In,Ru, V, Pd, Al, Ru and Ag.

Interestingly, it has also been found that EUV pellicles having a boroncore layer can be made much thicker for pellicles tuned for EUVwavelength of 6.7 nm. For example a 140 nm thick boron core layerprovides in itself around 90% EUV transmission without no need offurther cap layers for improved IR emissivity.

(Poly-)Si based pellicles which have been tuned via appropriate materialand membrane thickness choice to have a good EUV transmission (≥85%) mayhave an additional disadvantage that they reflect much of the DUVradiation potentially present in the exposure radiation spectrum and donot transmit DUV (i.e. they have high DUV reflectance and poor DUVtransmittance). They may also suffer from poor IR emissivity, althoughthe latter can be mitigated as described above for instance by adding acap layer for improved IR emissivity such as 1 to 2 nm Ru cap layer ontop of the pellicle core layer. Such Ru cap layer does not improve (orworsen) however the DUV reflectance and transmittance.

Besides lowering the DUV reflectance desired for better imaging, ahigher DUV transmittance can help to further lower the DUV radiationimpact at wafer level during EUV exposure, while also allowing DUV maskinspection.

Through pellicle inspection and high pellicle DUV reflection of EUVpellicles may be mitigated with a series of materials as shown below,which materials mitigate DUV reflection and in the same time enhance DUVtransmission at 157, 193 or 248 nm which are suitable wavelengths forknown mask inspection tools.

Several materials which allow for ArF, KrF and F₂ mask inspection toolsand less image degrading DUV at wafer are exemplified below:

-   -   Crystalline Yttrium has good 193 nm transmittance and low DUV        reflectance    -   (Poly-)crystalline Zr (e.g. ZrN and ZrC) and (poly-)crystalline        Y all have low DUV reflectance.    -   Amorphous and graphitic carbon-based pellicles may have good 157        and 193 nm transmittance and low DUV reflectance    -   Si₃N₄ pellicles may allow for 248 nm mask inspection at a still        low DUV reflectance.        All the above EUV pellicles also have good IR emissivity of more        than 0.2

It has been determined that crystalline Yttrium has a transmission peakat 193 nm and also has high IR emissivity. For example, a 20 nm thickyttrium core EUV pellicle covered on both sides with 1 nm Ru cap layerhas (in brackets a comparison is given with a Si+Ru equivalentpellicle):

-   -   DUV 193 nm transmittance of 67% (double pass 43%) (versus 0% for        Si+Ru)    -   DUV reflectance 100-250 nm <12% (versus 20-50% for Si+Ru)    -   DUV reflectance 250-400 nm <25% (versus >60% for Si+Ru)    -   EUV transmittance of 92.5% (versus 85% for Si+Ru)

Yttrium pellicles can be up to 50 nm thick for 90% EUV transmittance. Rucap layers applied on both sides of the EUV pellicle will limit thisupper thickness to about 36 nm. The thicker the yttrium core, the moreof 193 nm DUV radiation is lost.

It should be noted that to the present no other material with such high13.5 nm and 193 nm transmission has been found such as crystallineYttrium, which is a unique material in this respect.

Polycrystalline Yttrium does not have high 193 nm transmission. Howeverboth Zr-based and Y-based EUV pellicles have much lower DUV reflectionthan (poly-)Si. In fact, if for any reasons low DUV reflection is arequirement then (poly-)Si based pellicles may not be advantageous. If193 nm transmittance and through pellicle inspection are not needed,then polycrystalline Zirconium or Yttrium based pellicles with Ru caplayer(s) may also lower the DUV reflectance to much lower values thanfor (poly-)Si.

ZrN and ZrC may also have lower DUV reflectance than (poly-)Si. DUVtransmission of ZrN and ZrC is less than for Zr or Y, making howeverthrough pellicle inspection more difficult.

Crystalline, amorphous and graphitic carbon or carbon-based materialhave a DUV transmission peak at 157 nm and also high IR emissivity.Graphitic carbon is similar to multilayer graphene.

The reflectivity of MoSi multilayer mirrors is highest around 200-300nm. In this range DUV is best reflected to wafer (almost as good asEUV). All EUV pellicles described herein significantly reduce reflectionin this wavelength range. On the other hand (poly-)Si, SiC and(poly-)Si+Ru based pellicles are a worse choice for DUV reflection andtransmission. SiNx based pellicles may have better reflectivity above200 nm if the SiNx core is thin enough (e.g. 13 nm or less).

FIG. 12 compares the response of Si₃N₄ pellicles with Ru cap versus Sipellicles with Ru cap (see FIG. 12 showing the absorbance vs wavelength,wherein theoretical data (dashed lines) are compared to experimentalresults (solid lines)). Experiments with FTIR showed that Ru layers ofjust 2 nm with 3% EUV loss could enhance emissivity 400 fold from about0.001 to 0.4. Therefore a few nm Ru thick layer may enhanceabsorption/emissivity of a SiNx or Si membrane over 100 fold. The Si₃N₄pellicles (22 nm) were much thinner than the Si pellicles (60 nm) toensure sufficient EUV transmittance. It was found that Si₃N₄ basedpellicles have much lower DUV reflection and good DUV transmissionaround 250 nm. Si₃N₄ pellicles with 1-2 nm Ru cap layer reflect alsomuch less DUV radiation than Si+Ru pellicles, therefore it was foundthat pellicles based on Si₃N₄₊Ru cap layer could allow through pellicle248 nm KRF mask inspection.

Ru or other metal coatings are in principle not needed to enhanceemissivity for Zr and Y or graphitic/amorphous carbons. They may be usedhowever as protective cap layers to prevent for example oxidation. Anyother suitable (non-metal) protective cap layer that can preventoxidation of Zr and Y will also work well. Ru or other metal cap layersare preferably applied in case of Si₃N₄ and (poly-)Si for the purpose ofenhancing the IR emissivity.

Below there are some characteristics obtained by simulations given forseveral pellicles which have a good balance of emissivity, transmissionand absorption of EUV (13.5 nm), IR and DUV radiation:

Example 1: Ru coated crystalline Yttrium based Pellicles:

-   -   Have 90% EUV transmittance up to 35 nm thick    -   Have 70% 193 nm transmittance (193 nm inspection is a        possibility)    -   Have 2-5× lower DUV reflection than Si    -   Have emissivity close to 0.25        Example 2: Ru coated polycrystalline Yttrium or Zirconium based        Pellicles:    -   Have 90% EUV transmittance up to 25 nm thick (Zr) and 35 nm        thick (Y)    -   Have 10% 193 nm transmittance (193 nm inspection not possible on        Zr or Y pellicle)    -   Have 40% 248 nm transmittance (248 nm inspection may work in        case of Zr)    -   Have 2-3× lower DUV reflection than Si    -   Have emissivity close to 0.25    -   ZrC and ZrN based pellicles can also reduce DUV reflection by up        to factor of 2-8        Example 3: Ru coated Si₃N₄ (SiNx) pellicles (10 nm Si₃N₄        (SiNx)+2 nm Ru):    -   Have 90% EUV transmittance up to 10 nm thick (2 nm Ru)    -   Have 25% 193 nm transmittance    -   Have 70% 248 nm transmittance (248 nm inspection might work)    -   Have up to 10x lower DUV reflectance in range 200-400 nm    -   Have emissivity up to 0.5        Example 4: Amorphous and graphitic carbon (or multilayer        graphene):    -   Have 90% EUV transmittance up to 16 nm thick    -   Have 60-80% 157 nm transmittance (157 nm inspection is a        possibility)    -   Have 40-70% 193 nm transmittance (193 nm inspection is a        possibility)    -   Have 2-10× lower DUV reflectance    -   Have emissivity 0.15-0.4

FIG. 13 shows schematically another embodiment according to theinvention being a membrane assembly of two membranes transmissive to EUVradiation separated by a gap of thickness D, also referred to as a dualmembrane or dual EUV pellicle. Such a dual pellicle comprises two ormore EUV membranes of a material for improved IR emissivity asexemplified herein, for example two metal layers with a thickness lessthan the metal skin layer thickness of IR radiation. In such a dual EUVpellicle each individual metal layer is preferably chosen such thatthere is no EUV radiation reflection, since destructive interferencecannot be easily controlled for large distances.

FIG. 13 shows an embodiment where the layers for improved IR emissivityare Ru or Mo layers having a layer thickness D of 1-2 nm. The improvedIR emissivity layers of the dual pellicle may optionally each besupported by another support layer, such as a Si support layer, toprovide more mechanical strength. The EUV membranes are separated by agap D of 1-10 microns, preferable D is about 1-2 microns gap. Also amultiple membrane comprising alternating layers of metals andcorresponding gaps in the required thickness range are possible. Theadvantage of having a gap with the above-mentioned thickness D betweenthe EUV membranes (e.g. metal layers of thickness <metal skin layerthickness) is inducing IR resonance modes between the EUV membranes,which further enhance the IR emissivity of the membrane assembly.Another example of a dual EUV pellicle is 2× 10 nm ZrC or ZrB₂ separatedby 2 microns gap. Compared to lx 20 nm ZrC pellicle, the emissivity willbe increased from 0.45 to 0.7 (close to the theoretical limit of 1).

Typically the optimal emissivity of a pellicle obtained by doping(poly-)silicon or another semiconductor material is similar to that of apellicle coated with very thin layer of metal. Both cases may give up toabout 10× enhancement of IR emissivity. For example in the case ofdoping the maximum spectrally integrated IR emissivity obtained wasfound to be about 0.4, whereas in the case of applying a Ru cap layer of1 nm thickness, the maximum spectrally integrated IR emissivity obtainedwas found to be about 0.5.

In summary, this disclosure provides simple and robust examples forincreasing EUV membrane performance, and therefore performance of EUVpellicles and SPFs. EUV membrane temperatures, and temperature gradientsacross the EUV membrane, are reduced. As a consequence the lifetime ofthe EUV membrane and tolerance to EUV radiation power is improved.Additionally, high EUV membrane robustness is achieved withoutdecreasing EUV radiation intensities (deteriorating the manufacturingsystem performance).

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multilayerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims and clauses set outbelow.

1. A membrane transmissive to EUV radiation, comprising:

-   -   one or more high doped regions where the membrane is doped with        a high dopant concentration, and    -   one or more low doped regions where the membrane has no doping        or a low dopant concentration;    -   wherein a high dopant concentration is defined as dopant        concentration greater than 10¹⁷ cm⁻³ and a low dopant        concentration is defined as a dopant concentration less than        10¹⁷ cm⁻³.

2. A membrane according to clause 1, comprising a plurality of layerswhich include a main substrate and one or more additional layers,wherein:

-   -   the main substrate has a low dopant concentration and forms a        low doped region; and    -   the high doped regions are comprised within some or all of the        additional layers.

3. A membrane according to clause 2, wherein the additional layerscomprise one or more cover layers for the protection of the membranefrom etching or reacting agents, and the doped regions are comprisedwithin the cover layers.

4. A membrane according to clause 2, wherein the additional layerscomprise one or more cover layers and one or more intermediate layers,arranged such that an intermediate layer is located between a coverlayer and the main substrate; the cover layers being for the protectionof the membrane from etching or reacting agents material, and theintermediate layers having an intermediate lattice size between that ofthe main substrate and the cover layer so as to reduce stress within themembrane; and

-   -   wherein the high doped regions are comprised within the cover        layers and/or the intermediate layers.

5. A membrane according to any of clauses 2 to 4, wherein the mainsubstrate is comprised of a poly-Si material.

6. A membrane according to any of clauses 1 to 5, wherein the membrane,or a layer thereof, comprises a central region and a peripheral regionaround the central region, wherein the high doped region comprises thecentral region and the low doped region comprises the peripheral region.

7. A membrane according to any of clauses 1 to 6, wherein the membrane,or a layer thereof, comprises a plurality of the high doped regionsseparated by the low doped regions.

8. A membrane according to clause 7, wherein the separation betweenadjacent high doped regions is between 1 μm and 5 μm.

9. A membrane according to any of clauses 1 to 8, wherein the dopingconcentration is graded, and increases towards the center of themembrane, or a layer thereof.

10. A membrane according to any of clauses 1 to 9, wherein the highdoped regions are doped with a dopant concentration greater than 10¹⁸cm⁻³.

11. A membrane according to any of clauses 1 to 9, wherein the highdoped regions are doped with a dopant concentration greater than 10¹⁹cm⁻³.

12. A membrane according to any of clauses 1 to 9, wherein the highdoped regions are doped with a dopant concentration greater than 10²°cm⁻³.

13. A membrane according to any of clauses 1 to 12, wherein the lowdoped regions are doped with a dopant concentration less than 10¹⁶ cm⁻³.

14. A membrane according to any of clauses 1 to 12, wherein the lowdoped regions are doped with a dopant concentration less than 10¹⁵ cm⁻³.

15. A membrane according to any of clauses 1 to 12, wherein the lowdoped regions are doped with a dopant concentration less than 10¹⁴ cm⁻³.

16. A membrane according to any of clauses 1 to 15, wherein the membranehas a thickness less than 100 nm.

17. A membrane according to any of clauses 1 to 16, comprising aplurality of additional features on one or both surfaces of the membranewhich are operable to increase transverse heat transfer.

18. A membrane according to clause 17, wherein the additional featurescomprise ribs or wires extending normal from the membrane surface.

19. A membrane according to clause 17 or clause 18, wherein the distancebetween additional features is ≤1 μm.

20. A membrane according to any of clauses 17 to 19, wherein theadditional features are configured to resemble an echelette grating.

21. A membrane according to clause 20, wherein the additional featurescomprise repetitive groups of wires or ribs, with each group comprisingwires/ribs progressively descending or increasing in height.

22. A membrane according to clause 6, clause 7 or clause 8, comprisingonly a single layer.

23. A membrane according to any of clauses 1 to 22, wherein the highdoped regions are doped with a dopant material comprising one or moreof: S, Te, As, O, Al, Sn, Sb, In, Ga, Br, CI, I, C, B and N.

24. A membrane according to any of clauses 1 to 23, wherein the dopantis selected for N-type doping and the high dopant concentrationcomprises from (2 to 3)×10²⁰n/cm³ donor atoms.

25. A membrane according to any of clauses 1 to 24, wherein the dopantis selected for P-type doping and the high dopant concentration is atleast 4×10²⁰ n/cm³ acceptor atoms.

26. A membrane for a lithographic apparatus having IR radiationemissivity of at least 0.1 and being substantially transmissive to EUVradiation, comprising

-   -   a core layer of thickness 60 nm or less, the core layer        comprising a material substantially transparent for EUV        radiation selected from the list of from (poly-)Si, Si₃N₄, SiC,        ZrN, ZrB₂, ZrC, MoB₂, MoC, RuB₂, LaB₂, LaC, TiB₂, TiC,        (poly-)crystalline Yttrium, (poly-)crystalline Zr, Be, C, B and        B₄C, and    -   a cap layer for improved IR emissivity comprising a material        which absorbs IR radiation and having a layer thickness of 20 nm        or less.

27. A membrane according to clause 26, wherein the membrane has a caplayer—core layer—cap layer sandwich-like configuration.

28. A membrane according to clause 26, wherein the membrane has a corelayer—cap layer—core layer sandwich-like configuration.

29. A membrane according to any of clauses 26 to 28, further comprisingone or more other intermediate layers or cap layers.

30. A membrane according to any of clauses 26 to 29, wherein the corelayer is a multilayer stack comprising one or more layers of (poly-)Si,Si₃N₄, SiC, ZrN, ZrB₂, ZrC, MoB₂, MoC, RuB₂, LaB₂, LaC, TiB₂, TiC,(poly-)crystalline Yttrium, (poly-)crystalline Zr, Be, C, B and B₄C.

31. A membrane according to any of clauses 26 to 29, wherein the corelayer material is a composite material comprising a metal andnon-metallic EUV transparent impurities dispersed therein.

32. A membrane according to any of clauses 26 to 31, wherein the caplayer for improved IR emissivity is a metal layer.

33. A membrane according to clause 32, wherein metal cap layer has athickness which is less than the skin depth of the metal in IRradiation.

34. A membrane according to clause 33, wherein metal cap layer has athickness D =nλ/2, with n being an integer equal to 3 or more and λbeing a wavelength of the EUV radiation used for lithographic exposure.

35. A membrane according to clause 26, wherein the EUV membranecomprises two metal cap layers for improved IR emissivity separated by acore layer, the cap and core layers being arranged such that destructiveinterference of EUV radiation occurs on the two metal cap layers andthereby the resulting EUV reflection is zero.

36. A membrane according to clause 35, wherein the each metal cap layercomprise a 2 nm thick layer of Ru or Mo, and wherein the core layercomprises a (poly-)silicon layer of thicknesses selected from 8.4 nm,15.1 nm, 21.9 nm, 28.6 nm, 35.4 nm, 41.5 nm, 48.7 nm and 55.7 nm.

37. A membrane according to any of clauses 33 to 36, wherein the skindepth of the metal in IR radiation is about 10 nm.

38. A membrane according to any one of clauses 26 to 37, wherein thematerial of the cap layer for improved IR emissivity comprises a metalselected from Ru, Ti, Nd, Pr, Mo, Nb, La, Zr, B, Y and Be, wherein thecap layer is of a different material than the core layer.

39. A membrane according to any of clauses 26 to 37, wherein thematerial of the cap layer for improved IR emissivity comprises B₄C,SiNx, ZrO₂ or MoSi₂ and is of a different material than the core layer.

40. A membrane according to any of clauses 26 to 37, wherein thematerial of the cap layer for improved IR emissivity is a silicidedifferent than the core layer, such as ZrSi₂ or NbSi₂.

41. A membrane according to any of clauses 28 to 40, wherein the corelayer comprises (poly-)Si and the cap layer for improved IR emissivityis a Mo or Ru layer of thickness 5 nm or less.

42. A membrane according to any of clauses 26 to 37, wherein the corelayer comprises (poly-)Si and the cap layer for improved IR emissivitycomprises at least one of Ti, Nd, Pr, Nb, La, Zr, B, Y, Be, ZrO₂, MoSi₂,ZrSi₂ and NbSi_(2.)

43. A membrane according to any of clauses 26 to 40, wherein the corelayer comprises B, B₄C or Be and has a thickness of 25 nm or less.

44. A membrane according to clause 43, wherein the cap layer forimproved IR emissivity is a metal layer of thickness of 1-10 nm.

45. A membrane according to clause 26, wherein the core layer is amultilayer core comprising up to 20 pairs of B or B₄C and graphene,wherein the layer thickness ratio is 10 nm B or B₄C/3 nm graphene.

46. A membrane according to clause 26, wherein the core layer is amultilayer core comprising up to 20 pairs of SiNx and graphene, whereinthe layer thickness ratio is 10 nm SiNx/2 nm graphene.

47. A membrane according to clause 43 or clause 44, wherein the corelayer is a B or B₄C layer of thickness 5-15 nm and wherein the cap layerfor improved IR emissivity is a (poly-)crystalline Y, Ru or Mo layerwith thickness of 1-3 nm.

48. A membrane according to any of clauses 26 to 40, wherein the corelayer has a thickness of 16 nm or less and comprises a carbon-basedmaterial.

49. A membrane according to clause 48, wherein the carbon-based materialis a crystalline, amorphous or graphitic carbon layer.

50. A membrane according to clause 48 or clause 49, wherein the caplayer comprises a metal selected from Be, La, Te, Ti, Pr, Rh, Eu, In,Ru, V, Pd, Al. Mo, Zr, Nb and Ag.

51. A membrane according to any one of clauses 26 to 40, wherein thecore layer comprises silicon nitride and has a thickness of 15 nm orless.

52. A membrane according to clause 51, wherein the cap layer forimproved IR emissivity is a Ru or Mo layer of thickness 3 nm or less.

53. A membrane according to any of clauses 26 to 40, wherein the corelayer comprises (poly-)crystalline yttrium and has a thickness of 50 nmor less, preferably 35 nm or less.

54. A membrane according to any of clauses 26 to 40, wherein the corelayer comprises polycrystalline Zr and has a thickness of 25 nm or less.

55. A membrane according to clause 53 or clause 54, wherein the caplayer for improved IR emissivity is a Ru layer.

56. A membrane according to any of clauses 26 to 55, wherein the caplayer for improved IR emissivity is protected with a protective caplayer from a material protecting against oxidation and/or etchingselected from oxides, carbides or nitrides of the following materials:Zr, Ti, Hf, Si, Rh and Ru.

57. A membrane according to clause 56, wherein the protective cap layerhas a thickness from 1 to 3 nm.

58. A membrane according to any of clauses 26 to 40, wherein themembrane is transmissive for EUV radiation having the wavelength of 13.5nm, and wherein the core layer comprises at least one of ZrB₂, ZrC,MoB₂, MoC, RuB₂ or SiC.

59. A membrane according to any of clauses 26 to 40, wherein themembrane is transmissive for EUV radiation having the wavelength of 6.7nm, and wherein the core layer comprises at least one of ZrB₂, ZrC,LaB₂, LaC, TiB₂, TiC, MoB₂ or MoC.

60. A membrane according to any of clauses 26 to 40, wherein themembrane is transmissive for EUV radiation having the wavelength of 4.37nm, and wherein the core layer comprises TiC.

61. A membrane for a lithographic apparatus having IR radiationemissivity of at least 0.1 and being substantially transmissive to EUVradiation of 6.7 nm wavelength, the membrane comprising a core layerfrom a material comprising boron, wherein the core layer has a thicknessfrom 20 to 150 nm.

62. A membrane for a lithographic apparatus having IR radiationemissivity of at least 0.1 and being substantially transmissive to EUVradiation, the membrane comprising a core layer from a materialcomprising Ru, wherein the core layer has a thickness from 20 to 30 nm.

63. A membrane assembly for a lithographic apparatus having IR radiationemissivity of at least 0.1 and being substantially transmissive to EUVradiation, the membrane assembly comprising at least two metal caplayers for improved IR emissivity, the metal cap layers comprising ametal which absorbs IR radiation and have a layer thickness of 20 nm orless, wherein the metal cap layers for improved IR emissivity areseparated by a gap with thickness of 10 microns or less.

64. A membrane assembly according to clause 63, wherein the metal caplayers are each supported with a support layer which provides furthermechanical strength.

65. A mask assembly comprising a lithographic mask and a frame coupledto the mask, the frame being arranged to support a membrane or membraneassembly according to any of clauses 1 to 64.

66. A lithographic apparatus comprising one or more membranes or amembrane assembly according to any of clauses 1 to 65.

67. A lithographic apparatus according to clause 66, wherein at leastone of the membranes operates as a pellicle protecting a component fromcontamination.

68. The lithographic apparatus according to clause 67, furthercomprising a support constructed to support a patterning device, thepatterning device being capable of imparting a radiation beam with apattern in its cross-section to form a patterned radiation beam; whereinat least one of the membranes operates as a pellicle protecting thepatterning device from contamination.

69. The lithographic apparatus according to clause 67 or clause 68,further comprising a projection system operable to project a patternedradiation beam onto a wafer, wherein at least one of the membranesoperates as a pellicle protecting optical components within theprojection system from contamination.

70. A lithographic apparatus according to any of clauses 66 to 69,wherein at least one of the membranes operates as a spectral filtermembrane for blocking unwanted wavelengths of radiation.

71. A lithographic apparatus according to clause 70, wherein thespectral filter membrane is arranged under an angle in a scan directionof the lithographic apparatus, such that a radiation reflected by themembrane is not reflected back into the projection system.

72. A lithographic apparatus according to clause 70 or clause 71,wherein the spectral filter membrane for blocking unwanted wavelengthsof radiation comprises a metal layer having a thickness less than theskin depth for IR radiation and more than 5 nm.

73. A lithographic apparatus according to clause 72, wherein thespectral filter membrane comprises a metal substantially transparent toEUV radiation selected from Ru, Mo, La, Rh, Be, Y, Zr, Ce, Nb and Pr.

74. A lithographic apparatus according to clause 73, wherein thespectral filter membrane comprises a (poly-)Si core layer and a Ru or Mocap layer having a thickness from 5.5 to 10 nm.

1.-20. (canceled)
 21. A membrane for a lithographic apparatus, themembrane comprising a core layer of a thickness of 60 nm or less, thecore layer comprising a material, substantially transparent forradiation at an EUV wavelength, selected from: (poly-)Si, Si₃N₄, SiC,ZrN, ZrB₂, ZrC, MoB₂, MoC, RuB₂, LaB₂, LaC, TiB₂, TiC,(poly-)crystalline Yttrium, (poly-)crystalline Zr, Be, C, B or B₄C, anda cap layer for improved IR emissivity comprising a material whichabsorbs IR radiation and having a layer thickness of 20 nm or less,wherein combination of at least the core layer and cap layer of themembrane has IR radiation emissivity of at least 0.1 and issubstantially transmissive to radiation at the EUV wavelength.
 22. Themembrane as claimed in claim 21, wherein the material of the cap layerfor improved IR emissivity comprises Ru, Ti, Nd, Pr, Mo, Nb, La, Zr, B,Y or Be, and wherein the cap layer is of a different material than thecore layer.
 23. The membrane as claimed in claim 21, wherein thematerial of the cap layer for improved IR emissivity comprises B₄C,SiNx, ZrO₂ or MoSi₂ and is of a different material than the core layer.24. The membrane as claimed in claim 21, wherein the material of the caplayer for improved IR emissivity is a silicide different than the corelayer.
 25. The membrane as claimed in claim 21, wherein the core layercomprises (poly-)Si and the cap layer for improved IR emissivity is a Moor Ru layer of thickness 5 nm or less.
 26. The membrane as claimed inclaim 21, wherein the cap layer for improved IR emissivity is protectedwith a protective cap layer, the protective cap layer comprising amaterial protecting against oxidation and/or etching and the material isan oxide, a carbide or nitride of at least one selected from: Zr, Ti,Hf, Si, Rh or Ru.
 27. The membrane as claimed in claim 26, wherein theprotective cap layer has a thickness selected from the range of 1 to 3nm.
 28. The membrane as claimed in claim 21, wherein the membrane istransmissive for radiation having the EUV wavelength of 13.5 nm, andwherein the core layer comprises at least one selected from: ZrB₂, ZrC,MoB₂, MoC, RuB₂ or SiC.
 29. The membrane as claimed in claim 21, whereinthe membrane is transmissive for radiation having the EUV wavelength of6.7 nm, and wherein the core layer comprises at least one selected from:ZrB₂, ZrC, LaB₂, LaC, TiB₂, TiC, MoB₂ or MoC.
 30. The membrane asclaimed in claim 21, wherein the membrane is transmissive for radiationhaving the EUV wavelength of 4.37 nm, and wherein the core layercomprises TiC.
 31. A lithographic apparatus comprising the membrane asclaimed in claim
 21. 32. The lithographic apparatus as claimed in claim31, wherein the membrane operates as a pellicle protecting a componentfrom contamination.
 33. The lithographic apparatus as claimed in claim32, further comprising a support constructed to support a patterningdevice, the patterning device capable of imparting a radiation beam witha pattern in its cross-section to form a patterned radiation beam,wherein the membrane operates as a pellicle protecting the patterningdevice from contamination.
 34. The lithographic apparatus as claimed inclaim 32, further comprising a projection system operable to project apatterned radiation beam onto a substrate, wherein the membrane operatesas a pellicle protecting an optical component within the projectionsystem from contamination.
 35. The lithographic apparatus as claimed inclaim 31, wherein the membrane operates as a spectral filter membrane toblock an unwanted wavelength of radiation.
 36. The lithographicapparatus as claimed in claim 35, wherein the spectral filter membraneis arranged under an angle in a scan direction of the lithographicapparatus, such that a radiation reflected by the membrane is notreflected back into the projection system.
 37. The lithographicapparatus as claimed in claim 35, wherein the spectral filter membranecomprises a metal layer having a thickness less than the skin depth forIR radiation and more than 5 nm.
 38. The lithographic apparatus asclaimed in claim 37, wherein the spectral filter membrane comprises ametal substantially transparent to EUV radiation selected from Ru, Mo,La, Rh, Be, Y, Zr, Ce, Nb or Pr.
 39. The lithographic apparatus asclaimed in claim 38, wherein the spectral filter membrane comprises a(poly-)Si core layer and a Ru or Mo cap layer having a thickness from5.5 to 10 nm.
 40. A membrane for a lithographic apparatus having IRradiation emissivity of at least 0.1 and being substantiallytransmissive to radiation of 6.7 nm wavelength, the membrane comprisinga core layer made from a material comprising boron, wherein the corelayer has a thickness selected from 20 to 150 nm.